1. Field of the Invention
The invention relates to a method for reconstruction of a 3D volume from a set of recorded projection images in the field of dental medical applications.
2. Description of Related Art
Computed tomography (CT) is a digital 3D imaging method using penetrating radiation in particular X-ray radiation to obtain internal 3D information of an object. In a first recording step of CT a volume at least partially including the object is examined by irradiating the volume with the penetrating radiation from a number of different directions and recording corresponding projection images.
For this purpose, a tomography apparatus comprising a movable radiation source and a movable radiation detector is used. By moving the radiation source and the radiation detector along a generally circular scan trajectory around the object the tomography apparatus records a series of projection images wherein for each of the projection images the scan geometry is defined by the geometric relations between the volume including the object, the radiation source and the radiation detector.
The 3D volume, more precisely the localized radiation attenuation caused by object structures in the inspected volume, is then reconstructed with different mathematical CT reconstruction methods which generally take into account the scan geometry used for the recording of the projection images. After reconstruction the resulting 3D volume is presented to the user, e.g. as a series of 2D image slices cut from the 3D volume or by means of 3D visualization techniques, allowing insights into the inner structures of the examined object.
However, because of mechanical inaccuracies of the tomography apparatus, the assumed scan geometry used during reconstruction may deviate from the actual scan geometry resulting e.g. in a non-perfect approximately circular scan. Some of these mechanical inaccuracies can be compensated with the help of time consuming calibration procedures. Others might not be compensable. For example, the tomography apparatus may be afflicted with geometrical instabilities during the scan resulting in unpredictable variations of the scan geometry. As a result, the reconstructed 3D volume will show errors or artefacts which makes the interpretation by the user more difficult and might even lead to a wrong diagnosis.
A reconstruction method of computer tomography (CT) suggested by Kyriakou et al. in Phys. Med. Biol. 53 (2008) 6267-6289 therefore improves the reconstructed 3D volume by iteratively performing the reconstruction of the 3D volume using different values for the scan geometry parameters. According to the suggested method the quality of the reconstructed 3D volume is then improved by means of minimizing its entropy, which is a measure of the information content in an image. However, it is not generally guaranteed that lower entropy corresponds to a better reconstruction. Besides, this method only allows improvement with respect to a limited number of geometric parameters of the CT scan, whereas the degree of improvement is limited.